1. Field of Invention
The field of the present invention relates to providing for,
a. equality of the cell voltages of a battery during processes of discharge and charge. PA1 b. avoiding cell reversal during discharge under conditions of contemplated service use. PA1 a. to, insofar as feasible, equalize the state of charge of all cells by charging the battery to and beyond the point at which further charging does little to increase cell state of charge, and, PA1 b. to restrict depth of discharge, PA1 a. employ a d.c. to d.c. converter transformer turns ratio so chosen that only small reductions of the voltage of a cell below average cell voltage will result in flow of a consequential amount of converter output circuit current into the cell in question. PA1 b. employ d.c. to d.c. converters the cost of which has been reduced by designing them so that their rated allowable output current is lower than the planned for maximum current drain on the battery, an approach that is viable since a battery cell that loses charge so rapidly, or fails in the area of charge acceptance so greatly, that cell reversal cannot be prevented, can be dealt with by manually effected cell removal and replacement. Items (a) and (b) above stand in contrast to the Marshall Space Flight Center approach to battery protection in which, PA1 c. with average cell voltage at or above the 1.25 volts level converter no load output voltage is of the order of 0.8 to 0.85 volts (2), and flow of output circuit current into a defective cell, in other than wholly negligible degree, fails to take place until cell voltage drops to 0.7 (1,2) PA1 d. the protective scheme design concept has come to be to provide so that a battery will be able to deliver its maximum planned for discharge current even when one or several of its cells have opencircuited.
2. Description of the Prior Art
The usually employed method of equalizing battery cell state of charge, and preventing cell reversal on discharge, consists in providing,
processes which both waste energy, and reduce the effective energy storage capacity of the battery.
In the case of nickel-cadmium batteries a problem that is encountered in their use resides in the fact that, following on a sufficient number of cycles of discharge and recharge the voltage that is attained on charge begins to decrease, at which point it is customary to subject the battery to a process of prolonged discharge at low current, a procedure that is termed reconditioning. In ordinary uses of nickel-cadmium batteries, as for example in the case of batteries of aircraft type, the procedure is to first effect a partial discharge of the entire battery, and next, proceed to discharge each cell individually, by applying a resistor across its terminals.
When it comes to nickel-cadmium batteries planned for use in space vehicles, including earth satellites, the problem arises that it is not possible, or at any rate has hardly seemed to be practicable to provide to discharge cells individually as a last stage of a process of reconditioning, and this has meant that such reconditioning as has been provided for has had to be carried out via restricting to only partial discharge, since, with employment of full discharge it would have to be counted on that there would be a reversal of the potential of the weakest cells, with the net effect that those cells would be permanently damaged.
Following on the 1972 experience of difficulties with the Skylab Apollo Telescope Mount satellite, that had relation to battery performance, NASA's Marshall Space Flight Center initiated a program of development of a Ni-Cd battery cell reconditioning circuit (1) which had for its purpose rendering it possible to completely discharge one of two or more batteries with which a space vehicle is equipped without incurring cell reversal.
Subsequent to the 1976 publication of reference (1) the Marshall Space Flight Center carried out the design of several prototype Ni-Cd battery reconditioners, which, when tested, not only proved capable of performing as planned for, but also demonstrated that their employment rendered it possible to continue to employ a Ni-Cd battery as a power source even after one of its cells had developed a condition of open circuit, a finding that led to use of the term battery protective circuit in place of the term battery reconditioner.
In the Marshall Space Flight Center battery protective circuit use is made of a single input circuit d.c. to d.c. converter equipped with output circuits equal in number to the number of cells of the battery that is to be protected, which, in designs to date has been 22, or a multiple of 22, with the number of converters employed made equal to the total number of battery cells divided by 22.
The present invention represents a modified form of the Marshall Space Flight Center battery protective circuit in which the approach is to start from the basic method of battery protection that it embodies, and so modify it that it will be well adapted to serve as a way to protect batteries that are made use of on terra firma, and in aircraft, where, if need be, Ni-Cd batteries can be reconditioned by manually applying shorting resistors to individual cells, and a defective cell can be manually replaced.
Two key aspects of the writer's modified approach to battery protection are to,